There is an increased demand for monitoring cameras to operate in unmanned installations at night, and such cameras with zooming capabilities have recently been sought. Such a zoom optical system for a monitoring camera requires the ability to identify an object even in an environment of low illumination and that the zoom optical system is capable of providing a wide field angle (i.e., a wide-angle of view). However, it has also been increasingly desired that the zoom optical system be compact, have a high zoom ratio, and provide good optical performance throughout the entire range of zooming.
The wide-angle zoom lens described in Japanese Laid-Open Patent Application 2003-287677 is cited as an example of such a zoom optical system.
The zoom lens described in Japanese Laid-Open Patent Application 2003-287677 has a compact two-group construction with lens groups having negative and positive refractive power arranged in order from the object side. Such a construction, which has an object-side lens group of negative refractive power, has the strong point that it is suited to achieving a wide angle and ensuring an appropriate back focus distance. However, with this two-group construction of lens groups having negative and positive refractive power arranged in order from the object side, because the entire lens system becomes asymmetric and fluctuations in aberrations during zooming increase, if the lens system is to provide a bright image, maintaining good optical performance over the entire zoom range is a problem.
The zoom lens described in this Japanese application may be made small and with a large aperture ratio by arranging an aspheric lens on the image side of a lens element having positive refractive power and a convex surface on the object side of the second lens group.
Additionally, the zoom optical system described in Japanese Patent Application 2004-279168 has the same lens construction as that disclosed in Japanese Laid-Open Patent Application 2003-287677. This zoom optical system also satisfies certain conditions in order to achieve a wider field angle in addition to satisfying various other criteria discussed in Japanese Laid-Open Patent Application 2003-287677.
However, a demand to arrange monitoring cameras in a larger number of places has also increased in view of the needs of society for such monitoring cameras, thus increasing the demand for more inexpensive zoom optical systems with simpler construction for such monitoring cameras. Of course, in order to achieve this, there is no desire to sacrifice compactness, zoom ratio, or optical performance, but rather a desire that these features at least remain the same or be improved or increased.